U.S. patent number 5,284,631 [Application Number 08/002,126] was granted by the patent office on 1994-02-08 for crucible for manufacturing single crystals.
This patent grant is currently assigned to NKK Corporation. Invention is credited to Kenji Araki, Akio Fujibayashi, Takeshi Kaneto, Yoshinobu Shima.
United States Patent |
5,284,631 |
Kaneto , et al. |
February 8, 1994 |
Crucible for manufacturing single crystals
Abstract
A crucible including a cylindrical partition member arranged
concentrically therein for use in a silicon single crystal growing
apparatus. The bottom of the crucible located on the inner side of
the partition member has a thickness which is not less than 1.3
times and not greater than 4 times the thickness of the partition
member and it also has a porosity which is between 0 and 0.2% in
its inner layer and between 0.2 and 15% in its outer layer as
compared with the porosity of the partition member which is 0.2% or
less. By virtue of the foregoing, a D.F. ratio (ratio of
dislocation free) of 80% or over can be expected.
Inventors: |
Kaneto; Takeshi (Tokyo,
JP), Fujibayashi; Akio (Tokyo, JP), Shima;
Yoshinobu (Tokyo, JP), Araki; Kenji (Tokyo,
JP) |
Assignee: |
NKK Corporation (Tokyo,
JP)
|
Family
ID: |
26669982 |
Appl.
No.: |
08/002,126 |
Filed: |
January 7, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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816814 |
Jan 3, 1992 |
|
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Current U.S.
Class: |
117/213;
117/900 |
Current CPC
Class: |
C30B
15/12 (20130101); Y10T 117/1052 (20150115); Y10S
117/90 (20130101) |
Current International
Class: |
C30B
15/10 (20060101); C30B 15/12 (20060101); C30B
035/00 () |
Field of
Search: |
;156/617.1,618.1,619.1,620.1,620.4,DIG.64 ;422/248,249 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kunemund; Robert
Assistant Examiner: Garrett; Felisa
Attorney, Agent or Firm: Meller; Michael N.
Parent Case Text
This application is a continuation of application Ser. No. 816,814,
filed Jan. 3, 1992, now abandoned.
Claims
What is claimed is:
1. A silicon single crystal manufacturing apparatus comprising a
crucible containing molten silicon and a partition member arranged
inside said crucible in contact with a bottom portion of said
crucible for forming a single crystal growing section from which a
single crystal is pulled, said bottom portion of said crucible
having a thickness which is not less than 1.3 times and not greater
than 4 times the thickness of said partition member and a porosity,
at least where said bottom portion is in contact with molten
silicon, of greater than the porosity of said partition member with
said partition member having a porosity of not greater than 0.2%
and at least one hole in a lower part thereof.
2. A silicon single crystal manufacturing apparatus as defined in
claim 1, wherein the crucible is composed of a quartz silica
glass.
3. A silicon single crystal manufacturing apparatus as defined in
claim 2, wherein the crucible bottom portion of said single crystal
growing section comprises an inner layer in contact with said
molten silicon and an outer layer said inner layer having a
thickness of between 2 mm and 13 mm and a porosity of not less than
0.2% and not greater than 15%.
4. A silicon single crystal crucible according to claim 3, wherein
said inner layer of said crucible bottom portion in said single
crystal single crystal growing section and said partition member
are made of the same silica glass.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silica glass crucible used for
the manufacture of silicon single crystals according to the
Czochralski method.
2. Description of the Prior Art
The method of manufacturing silicon single crystals according to
the Czochralski method has heretofore been in use and it has become
substantially a perfect technique.
As is well known, this technique is so designed that after molten
silicon starting material has been contained in a silica glass
crucible, a silicon seed crystal is brought into contact with the
molten silicon surface and simultaneously the silicon seed crystal
is slowly pulled while rotating it, thus growing a silicon single
crystal along with the solidification at the contact surface
between the pulled silicon single crystal and the molten silicon
surface and thereby producing the cylindrical silicon single
crystal.
At this time, in order that the silicon single crystal may become a
P-type or N-type semiconductor in accordance with the intended use,
the silicon starting material is mixed with a suitable amount of
such dopant as boron, antimony or phosphorus. The ratio of such
dopant taken into the crystal from the molten silicon (i.e., the
segregation coefficient) is generally not greater than 1. The
concentration of the dopant in the silicon single crystal
determines its resistivity and therefore it should preferably be
uniform in the crystal.
Also, besides the dopant which is intentionally introduced into the
silicon single crystal as mentioned above, the presence of oxygen
introduced unavoidably during the manufacture is not negligible. In
other words, the concentration of the oxygen which is introduced
into the silicon single crystal has a considerable influence on the
characteristics and yield of a semiconductor product and the oxygen
concentration should also be uniform throughout the upper part to
the lower part of the single crystal.
However, as the pulling of the silicon single crystal proceeds, the
molten silicon with the crucible is decreased and the concentration
of the impurity is varied. In other words, since the segregation
coefficient of the dopant is not greater than the dopant
concentration of the molten silicon is increased gradually so that
the silicon single crystal is varied in dopant concentration from
the upper part to the lower part of the crystal. Also, since the
oxygen concentration of the molten silicon is dependent on the
amount of the oxygen released from the silica glass crucible into
the molten silicon, the concentration of the oxygen introduced into
the crystal is also varied with decrease in the molten silicon.
As mentioned previously, the quality of the pulled silicon single
crystal is varied along the pulling direction. However, the product
actually used as wafers is limited to the portions having dopant
concentrations and oxygen concentrations in limited ranges. As a
result, the extent of the pulled silicon crystal which can be used
as the product is extremely limited.
Some methods have been proposed for the solution of these problems
and the typical method which can be considered as practical is one
employing a double-structure crucible.
In other words, Japanese Patent Publication No. 40-10184 discloses
a method in which a concentric crucible adapted to be heated from
the outer periphery thereof is so constructed that the molten
silicon in an outer crucible is separated from the molten silicon
on the inner side by a partition but the two are mutually
communicated and a semiconductor crystal is pulled centrally while
feeding the semiconductor starting material to the molten silicon
on the outer side.
Referring to FIG. 10, there is schematically shown a silicon single
crystal manufacturing apparatus employing a double structure
crucible, and a crucible 22 and a partition member 23 are
constructed integrally by using high-purity silica glass. Numeral
25 designates the molten silicon contained in the crucible 22, and
26 a silicon single crystal pulled from the surface of the molten
silicon within the partition member 23. It is to be noted that the
lower part of the partition member 23 is formed with a hole 24 to
permit the molten silicon 25 to flow between the outer and inner
sides of the partition member.
FIG. 10 is a schematic diagram showing the case in which the double
structure crucible is incorporated in a batch-type silicon single
crystal manufacturing apparatus. The molten silicon of a given
dopant concentration is contained on the inner side of the
partition member 23 and the molten silicon containing no dopant is
contained on the outer side of the partition member 23. It is
constructed so that a silicon single crystal 26 is pulled from the
single flows to the single crystal growing section from the outer
side of the partition member, hereby always maintaining uniform the
concentration of the dopant within the single crystal growing
section.
Referring now to FIG. 11, there is illustrated another type of
construction in which while pulling a silicon single crystal from
the single crystal growing section, powder starting material 29 is
continuously fed to the starting material feed section from a
starting material feed pipe 28 thereby maintaining constant the
amount of the molten silicon within the single crystal growing
section, and it has the object of maintaining constant the dopant
and oxygen concentrations of the molten silicon in the single
crystal growing section.
Where the double structure crucible is used to pull a silicon
single crystal in accordance with such conventional technique, the
heat environment in the molten silicon becomes directly opposite to
the case in which the ordinary single structure crucible is
used.
In the case of the CZ method using the ordinary single structure
crucible, the crucible side wall portion is higher in temperature
than the crucible bottom portion. In other words, the amount of
heat supplied from the crucible side wall portion is greater than
the amount of heat supplied from the crucible bottom portion.
Reflecting this fact, it is said that the convection of the molten
silicon within the silica glass crucible is predominated by the
flows as shown in FIG. 8. If such convection of the molten silicon,
there is less temperature variation at the solid-liquid interface
between the silicon single crystal and growth is attained.
Where the pulling of a silicon single crystal is effected by using
the double structure crucible, however, the amount of heat supplied
to the single crystal growing section through the crucible side is
supplied indirectly through the starting material feed section and
therefore the proportion of the heat input through the crucible
bottom is increased as compared with the case where the
single-structure crucible is used. As a result, the temperature
distribution in the double structure crucible becomes opposite to
the case where the single structure crucible is used and therefore
the maximum value of the temperature in the silica glass crucible
surrounding the single crystal growing section is positioned at the
crucible bottom portion. Its temperature distribution is such that
the crucible bottom portion is high in temperature and the
partition member wall surface is relatively low in temperature.
Under such heat environment where proportion of the heat input
through the bottom portion is large, the heat convection of the
molten silicon within the single crystal growing section may
possibly be predominated by the flow field as shown in FIG. 9 which
is directly opposite to that shown in FIG. 8. Since such flow field
is unstable, the high-temperature molten silicon at the crucible
bottom portion is intermittently moved directly to the solid-liquid
interface of the silicon single crystal so that the resulting heat
variation causes defects in the silicon single crystal to be pulled
and hence the occurrence of dislocations.
On the other hand, the pores included in the silica glass crucible
may also be considered as a cause for impeding the stable pulling
of the silicon single crystal. In other words, while the surface of
the silica glass crucible is subjected to erosion by its reaction
with the molten silicon, at this time the pores confined within the
silica glass crucible break and enter into the molten silicon thus
giving rise to a problem that dislocations are caused when the
resulting bubbles and the broken pieces of silica glass the
solid-liquid interface of the silicon single crystal.
SUMMARY OF THE INVENTION
The present invention has been made with a view to overcoming the
foregoing problems in the art and it is an object of the invention
to provide an improved double structure crucible in which the
material and thickness at its various parts are optimized so that
the heat environment of molten silicon within a single crystal
growing section is improved and simultaneously the occurrence of
bubbles within the single crystal growing section is reduced,
thereby attain into the stable pulling of a silicon single
crystal.
The bottom of the crucible portion surrounded by the partition
(i.e., the single crystal growing section) is designed to have a
thickness which is not less than 1.3 times and not greater than 4
times the thickness of the partition member and also the porosity
of the single crystal growing section bottom is greater than the
porosity of the partition member. Also, the partition member and
the inner surface of the crucible bottom portion of the single
crystal growing section are constructed with the same silica glass
member having a low porosity not greater than 0.2%.
The amount of heat supplied into the single crystal growing section
is determined by the properties and shape of the silica glass
member which surrounds it. The crucible bottom portion of the
single crystal growing section is increased in thickness for the
purpose of reducing the heat input through the crucible bottom
portion by utilizing the fact that the heat conductivity of silica
glass is not good. In the case of the double structure crucible, if
all the parts of the crucible have the same thickness as in the
past, the heat flux through the crucible bottom portion becomes
excessively large and the heat flux through the partition member
becomes excessively small as compared with the case in which the
ordinary single structure crucible is used. The reason is that the
molten silicon on the outer side of the partition member impedes
the heat flow through the side heating zone.
The thickness of the crucible bottom portion of the single crystal
growing section is selected not less than 1.3 times of that of the
partition member on the ground that the heat environment of the
ordinary single structure crucible cannot be realized if the value
is not greater than 1.3 times.
However, the heat environment of the CZ Method (the single
structure crucible) cannot be attained by merely considering the
thickness of the crucible as mentioned above. Assuming that the
bottom portion is increased in thickness, the effect of the
increased thickness will be reduced greatly if its material is a
good heat conductor the amount of heat which enters and leaves
through the silica glass includes one due to the heat conductivity
and another that is transmitted as a radiant heat. The heat
conductivity of the silica glass is on the order of several
Kcal/m.h.k. and the heat conductivity is deteriorated with increase
in the non-transparency of the silica glass. On the other hand, the
transmission of the heat as a radiant heat has a greater dependency
on the transparancy of the silica glass and the rate of scattering
of the radiant heat is increased with increase in the porosity Of
the silica glass, thereby decreasing the transmission. While the
heat conductivity can be deteriorated by increasing the bottom
portion in thickness, if the bottom portion is transparent to the
radiant heat, the effect of restraining the amount of heat
transmitted is reduced. The average porosity of the crucible bottom
portion of the single crystal growing section is selected to be
equal to or higher than that of the partition member in
consideration of the foregoing points.
However, it is said that if the crucible bottom portion is made of
the silica glass having a high porosity, when the silica glass is
heated, the pores existing within the silica glass are expanded so
that as the melting of the quartz surface proceeds, the bubbles and
the broken pieces of the silica glass are discharged into the
molten silicon thus causing the occurrence of dislocations in the
silicon single crystal. The inner surface of the crucible bottom
portion of the single crystal growing section is made of the low
porosity silica glass for the purpose of avoiding such
phenomenon.
In addition, the thickness of the bottom of the single crystal
growing section is selected to be not greater than 4 times the
thickness of the partition member on the ground that if the
thickness of the impossible to satisfactorily melt the silicon
starting material loaded into the crucible at the start of the
operation.
The porosity of the partition member is selected to be not greater
than 0.2% for the purpose of facilitating the heat input to the
single crystal growing section through the side and thereby
ensuring the required amount of the heat input. In other words,
since the amount of heat passed through the partition member is
reduced if it is made of the silica glass having a high porosity,
the thickness of the crucible bottom portion must be further
increased correspondingly and it also becomes impossible to ensure
the required amount of the heat input to the single crystal growing
section.
In the method of pulling a silicon single crystal by using a
silicon single crystal growing silica glass crucible in which a
partition member is arranged internally so as to concentrically
surround a crystal and the partition member is formed with a small
hole to permit the movement of the molten silicon from the
outerside to the inner side of the partition member, the
thicknesses of the various parts of the silica glass crucible are
optimized so that the heat environment within the single crystal
growing section improved and the heat convection of the molten
silicon is stabilized.
On the other hand, since the heat transmission of the silica glass
component is practically dependent on the porosity of the silica
glass and the heat transmission is increased with decrease in the
porosity, by selecting the porosity of the partition member to be
lower than the porosity of the crucible bottom portion, it is
possible to increase the amount of heat input through the crucible
side (the partition member) as compared with the bottom of the
crucible.
Further, by decreasing the porosity of the surface of the silica
glass member surrounding the single crystal growing section, it is
possible to prevent the occurrence of bubbles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1, 2 and 3 are sectional views showing schematically
embodiments of the present invention.
FIG. 4 is a sectional view showing schematically an example of a
silicon single crystal pulling apparatus used for incorporating the
present invention.
FIGS. 5, 6 and 7 show the results of the experiments on the
embodiments of the present invention.
FIGS. 8 and 9 are diagrams respectively showing the convection of
molten silicon in cases where a single structure crucible is used
and where a conventional double structure crucible is used.
FIG. 10 is a diagram showing an example of a silicon single crystal
manufacturing method employing a double structure crucible.
FIG. 11 is a diagram showing another example of the silicon single
crystal manufacturing method employing a double structure crucible.
In the drawings:
Numeral 1 designates a crucible, 2 a graphite crucible, 3 silicon
starting material, 4 molten silicon, 5 a silicon single crystal, 6
a heater, 8 a chamber, 11 a partition member (inner crucible), 12 a
small hole, 30 a crucible side wall portion, 31 a partition member,
32 a crucible bottom portion, 35 a large-diameter crucible, and 36
a small-diameter crucible.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a sectional view showing schematically an embodiment of
the present invention. In the Figure numeral 30 designates the side
wall of a quart crucible, 31 a partition member, 32 the crucible
bottom portion of a single crystal growing section.
The partition member 31 is made of a low porosity silica glass
having a porosity of not greater than 0.2% and both of the side
wall 30 and the crucible bottom portion 32 are made of the ordinary
silica glass having a porosity of 0.2% to 15%. The reason for
selecting the upper limit of the porosity to be 15% is that if it
is greater, the possibility of causing dislocations in the crystal
during its pulling becomes extremely high. The thicknesses of the
various parts of the crucible are selected so that as for example,
the crucible side wall 30 is 10 mm, the partition member 31 is 10
mm and the crucible bottom portion is 20 mm.
FIG. 4 is a sectional view of an apparatus on the whole which is
used for pulling a silicon single crystal by use of such silica
glass crucible as described above. Numeral 1 designates a silica
glass crucible set in a graphite crucible 2 which in turn is
vertically movable and rotatably supported on a pedestal 3. Numeral
4 designates silicon starting material contained in the crucible 1
so that a silicon single crystal 5 grown into a cylindrical shape
is pulled from the silicon starting material 4. Numeral 6
designates a heater surrounding the graphite crucible 2, and 7 a
hot zone heat insulator surrounding the heater 6, with these
components being basically the same with the single crystal
manufacturing apparatus according to the ordinary Czochralski
method.
With this apparatus, when the double structure silica glass
crucible as shown in FIG. 1 is used, the amount of heat supplied
through the bottom portion of the single crystal growing section is
restrained and contrary the amount of heat supplied through the
side portion (the partition member) is increased, thereby ensuring
the stable convection of the molten silicon.
FIGS. 5 and 6 show the results of the experiments conducted by the
inventors to confirm the effect of the embodiment as shown in FIG.
1. FIG. 5 shows the relation between the thickness of the crucible
bottom portion and the D.F. (dislocation free) ratio in a case
where the partition member and the partition outer side portion are
both 10 mm in thickness, the porosity of the partition member is
0.2% and the porosity of the crucible bottom portion is varied to
0.1, 0.2, 0.5 and 1.5%, respectively. Ratio of D.F. (Ratio of
Dislocation Free) is defined as a probability (acceptance) of
pulling without defect, silicon crystal over the length of 1 m
during the work of pulling thereof. That is, when the accepted
product of defect free silicon crystal over the length of 1 m
during 10 times work is 5 times, the Ratio of D.F. is 50%.
As will be seen from FIG. 5, the presence of a problem has been
confirmed that if the thickness of the crucible bottom portion is
not greater than 13 mm, there is no restraining effect on the heat
input through the crucible bottom portion thus failing to attain
the stable pulling of a silicon single crystal, whereas if the
thickness is not less than 40 mm, the heat input through the
crucible bottom portion is reduced excessively thus failing to melt
the starting material silicon at its heating and melting stage.
Also, where the porosity is 0.1 which is less than that of the
partition member, the stable pulling is not attainable even if the
crucible thickness is not less than 13 mm.
FIG. 6 shows the relation between the porosity of the partition
member and the D.F. ratio in a case where the partition member and
the partition outer side portion have the same thickness of 10 mm,
the thickness of the crucible bottom portion is 13 mm and the
porosity of the crucible bottom portion is varied to 0.1%, 0.5% and
0.2%, respectively. As will be seen from the Figure, it has been
found out that if the porosity of the partition member exceeds the
average porosity of the crucible bottom portion, the stable pulling
of a silicon single crystal is impeded. Also, while essentially the
stable pulling is possible if both the partition member and the
crucible bottom portion have the same porosity of 0.5%, in cases
where the porosity is not less than 0.2%, the long-term pulling of
30 hours or over causes deterioration of the silica glass thus
tending to cause dislocations in the crystal.
The embodiment of FIG. 2 is intended to overcome the foregoing
problems and it is designed so that the outer surface of the
crucible bottom portion of the single crystal growing section is
made of a high porosity silica glass and its inner surface is made
of a low porosity silica glass.
FIG. 7 shows the results of the experiments conducted to confirm
the effect of the embodiment of FIG. 2. FIG. 7 shows the relation
between the porosity of the crucible bottom inner surface and the
D.F ratio in a case where the long-term pulling (over 30 hours) is
effected by selecting the thicknesses of the partition member and
the crucible outer side portion 10 mm, selecting the thickness of
the crucible bottom portion 20 mm, varying the porosity of the
partition member to 0.2% and 0.5%, respectively, and selecting the
porosity of the crucible bottom outer side 1.2%. As will be seen
from the Figure, it has been confirmed that if the porosity of the
crucible bottom inner surface is not greater than 0.2%, the stable
pulling of a silicon single crystal is attainable in the long-term
pulling. Further, if the low porosity layer on the inner surface of
the crucible bottom portion is not greater than 2 mm, there is the
possibility of this layer being partially melted and lost and the
stable pulling cannot be performed.
The upper limit is selected to be 13 mm on the ground that if it is
made thicker than this, there is the danger of excessively
increasing the heat input through the bottom. It is to be noted
that in the case of the two layer construction, the porosity of the
crucible bottom can be defined as the average value of the
porosities of the inner surface and the outer side and the
discussions of FIGS. 5 and 6 hold on the basis of this average
value.
The fusing operation of the double structure crucible as shown in
FIG. 2 is performed as a part of the crucible manufacturing
operation and the fusing of the silica glass members at various
parts can also be effected by the amount of heat supplied during
the melting of the silicon starting material. The embodiment of
FIG. 3 comprises a double structure crucible constructed by using a
large-diameter crucible having a high porosity and a small-diameter
crucible 36 having a low porosity and fusing the bottom outer
surface of the small-diameter crucible and the bottom inner surface
of the large-diameter crucible to each other, thereby obtaining
substantially the same effect as the embodiment of FIG. 2.
As will be seen from the foregoing description, the present
invention is such that in the silicon single crystal manufacturing
method employing the double structure crucible the amount of heat
supplied through the side of the single crystal growing section is
increased over that supplied through the bottom so that not only
the stable heat convection of the molten silicon is ensured but
also the occurrence of bubbles from the silica glass is prevented,
thereby attaining the stable pulling of a silicon single crystal.
Thus, working of the present invention has a great effect.
* * * * *